Single point imaging is a subset of volumetric imaging techniques within the discipline of magnetic resonance imaging as taught by S. Subramanian et. al. entitled “Single-Point (Constant-Time) Imaging in Radiofrequency Fourier Transform Electron Paramagnetic Resonance” published in Magnetic Resonance in Medicine. It typically contains three main components (see FIG. 1): 1) a broadband radio frequency (RF) excitation pulse (100) that excites a polarized species (110 or 120); 2) at least one gradient pulse that adds a phase dispersion across the sample (130 and 140); and 3) an acquisition window that measures radio frequencies near to the Lamour frequencies of the excited sample (150).
FIG. 1 illustrates a typical single point imaging pulse sequence. A broadband or slice selective RF pulse, 100, excites the spins of a polarized species. These species can have various T2 decay times as illustrated by species 110 having a quicker decay time compared to species 120. Usually but not limited to after the RF pulse, a phase encoding gradient 130 and 140 may be applied in at least one or a multitude of directions. After a delay, the signal from the decaying signal is acquired by using a RF coil in the time window as shown by 150. During this acquisition time, at least one encoding gradient is still active.
Single point imaging can be used to image materials with very short T2* decay times (approximately less than 300 microseconds as illustrated by 110); however, it has also been used as a technique to improve image signal to noise, and reduce field inhomogeneity artifacts. The inherent tradeoff to this imaging sequence is the large amount of time needed to image the entire k-space region compared to the quick imaging time of echo planar imaging, fast gradient echo sequences, or fast spin echo sequences.
Single point imaging schemes may be used to image materials with very fast T2-decay times such as dense solids because the minimum echo time can be close to zero. This short echo time further enables the ability to use such pulse sequences to image magnetic nanoparticles. Introduction of such magnetic materials may decrease the T2*-decay time of the surrounding media, e.g., by dephasing a surrounding water signal by the time that conventional imaging sequences take to acquire data. Other pulse sequence strategies, such as sweep imaging with Fourier transform (as taught by C. Corum, et. al in the article entitled “Introduction to SWIFT (Sweep Imaging with Fourier Transformation) for Magnetic Resonance Imaging of Materials” published in the Materials Research Society Symposium Proceedings), have been employed to visualize both solid and metallic structures but these require complex pulse sequence shapes and swept radiofrequency transmission schemes.
To ease hardware requirements and decrease acoustic noise, the most common variants of single point imaging schemes have a constantly powered magnetic gradient that is ramped through the entire range of applied gradient strengths over the imaging sequence. These have been termed single point ramped imaging with T1-enhancement or SPRITE imaging and are discussed by M. Halse et al. in a paper entitled “Centric scan SPRITE magnetic resonance imaging” published in Journal of Magnetic Resonance or by C. Kennedy, et. al. in a paper entitled “Three-dimensional magnetic resonance imaging of rigid polymeric materials using single-point ramped imaging with T1 enhancement (SPRITE)” published in the Canadian Journal of Chemistry. While SPRITE imaging schemes decrease eddy currents within the sample, decrease acoustic noise, and relax the requirements on the gradient hardware, SPRITE has a similar tradeoff that to traditional single point imaging strategies: the encoding gradients remain powered during at least part of the signal acquisition window causing increased dephasing of the signal during the entire readout window.